The biochemical mechanisms by which PALA, L-alanosine and 5-fluorouracil produce toxicity were examined: 1. For PALA, six mechanisms were explored: differences in transport, in enzyme kinetics, in catabolism of PALA, in the activity of the enzymes of pyrimidine salvage, in the rate of pyrimidine catabolism, and in the specific activity of L-aspartate transcarbamylase in vivo The best correlation was observed with the last parameter: tumors and tissues with low levels of the transcarbamylase were susceptible to PALA. 2. In the case of L-alanosine, the rate at which the effector metabolite is formed and destroyed in a given tissue or tumor was correlated with susceptibility to the drug. In two lines of tumor cells, one sensitive and gne resistant to alanosine, no marked difference in the specific activity of the anabolizing enzyme, (SAICAR kinosynthase) was observed. However, the catabolizing enzyme, adenylosuccinate lyase was elevated in the two sensitive tumors. 3. In the case of 5-FU, the ability of extracts of cancers to form and destroy the inhibitory species, FUTP and FdUMP, has been examined in vitro. Those tumors richest in enzymes phosphorylating FU and deficient in the phosphates which destroy the resultant anabolite were most susceptible.